1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a disk drive that includes a system for canceling the effects of rotational vibration.
2. Description of the Related Art
Magnetic recording hard disk drives use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data.
Disk drives experience rotational vibration and disturbance forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system.
Rotational vibration (RV) cancellation is a method that uses sensors to detect rotational vibration and improve the PES by canceling the off-track motion induced by the rotational vibration. One approach uses two linear accelerometers or vibration sensors, typically mounted on the disk drive's printed circuit board, to measure the RV. The measured RV is input to a feedforward controller that creates a feedforward compensation signal that is summed with the control signal to the VCM actuator. This method is sometimes called RV feedforward (RVFF). For RVFF to function effectively and not create an undesirable compensation signal that degrades the PES, the two linear vibration sensors should have minimal gain mismatch, i.e., minimal mismatch in their primary-axis sensitivities and minimal off-axis sensitivities, in addition to minimal mismatch in their orientations on the circuit board. In addition, it is also required that the circuit board have sufficiently high rigidity and damping so there is no significant standing or propagating waves on the circuit board to cause false sensor output (noise) that is not related to the RV. Thus the manufacturing cost of the disk drive and the component costs of the linear vibration sensors and circuit board can be significantly increased if it is desired to use RVFF without also degrading the PES and thus the performance of the disk drive.
U.S. Pat. No. 6,952,318 B1 describes an RVFF method that addresses mismatch in primary-axis sensitivities of the two linear vibration sensors by converting the two sensor outputs into separate digital signals and scaling them. The method requires inducing self-generated vibration in the disk drive and then comparing the magnitude of the two sensor outputs to reduce the difference in the gains of the two sensors. However, self-generated vibration in disk drives is normally not pure linear vibration but contains a significant amount of RV. Depending on the center of the RV, it is normally expected that the magnitudes of the two sensor outputs would be different, even with exactly matched primary-axis sensitivities. Also, this RVFF method does not address the problem of noise caused by off-axis sensitivity and circuit board vibration.
U.S. Pat. No. 7,035,034 B2 describes an RVFF method to determine the optimal gains for the two linear vibration sensors by first disabling RVFF and then collecting data and calculating the optimal gains, which then become fixed values for use in the RVFF.
What is needed is a disk drive with an effective RVFF method that does not require linear vibration sensors with exactly matched gains or a circuit board with special rigidity and damping characteristics, wherein the sensor gains are adaptive so as to be optimal under any given condition during operation of the disk drive.